The biggest reason for a rear inlet is to keep things simple. Most of us and most manufactures like to have the fan enclosed in the back of the burner with all the other mechanical parts of the burner so everything is packaged nicely and access is easy. If the inlet is in the back then there is less routing and less chance of future failure. As far as a low inlet, this could complicate the process. A high inlet provides two major import aspects.1 Even air distribution around the charcoal bed, or more properly described as the "hearth zone." 2 preheating of the air before it reaches the hearth zone as it passes through the reduction zone. If your air is too cool when it hits the hearth zone it will kill efficiency drastically. Likewise, an uneven distribution of air will cause bridging in The hearth zone and inhibit proper gassification. Bridging is when the coals burn faster then the wood above reduces to charcoal and gassification stops completely. You are on the right track when it comes to secondary mixing, this can be very critical to achieve proper and complete burn, but be wary of too much primary chamber pressure and too much nozzle velocity as this too can cause bridging. Size your nozzle to your heat needs, and make an air flow adjustment for the primary chamber. The ideal nozzle velocity for wood is 33 feet per second. This will determined e how many nozzles and their size. A 48" chamber is not to much, but keep in mind you want all the wood to get to the nozzles on its own so design accordingly. Use gravity here. If you need some help with designing the nozzle or calculations just ask, time is a little tight for me lately but ill be happy to help as much as I can.